You're as useless as... insults. Not everyone's as useful as you so here's some ammunition to tell someone exactly how useless they are.
You're as useless as a concrete parachute.
You're as useless as a glass hammer.
You're as useless as a wooden frying-pan.
You're as useless as a knitted condom.
You're as useless as ejection seats on a helicopter.
You're as useless as rubber lips on a woodpecker.
You're as useless as a screen door on a submarine.
You're as useless as a one-legged man at an arse kicking contest.
You're as useless as the G in Lasagna.
You're as useless as a ham sandwich at a Barmitzvah.
You're as useless as a Red Lights in Grand Theft Auto
You're as useless as Anne Frank's drum kit.
You're as useless as a chocolate teapot.
You're as useless as the 'ay' in 'okay'
You're as useless as a color blind interior decorator.
You're as useless as an ashtray on a bike.
You're as useless as a grave robber in a crematorium.
You're as useless as condom machines in the Vatican.
You're as useless as a cup of decaf.
You're as useless as handles on a snowball.